First principles study of the adsorption of hydrated heavy metals on graphene quantum dots

The adsorption of hydrated heavy metals on graphene quantum dots is investigated using the density functional theory. The considered heavy metals are Cd and Pb that are hexa-, penta-, and tetrahydrated with water molecules. Various adsorption schemes are considered such as surface and edge adsorption, and the interaction with functional groups attached to the flake's edges. The calculated positive adsorption energy implies that the considered graphene flakes are able to adsorb the hydrated heavy metals through all the proposed positions and interactions. Both physical and chemical adsorption have been observed, the physical adsorption is characterized by longer adsorption distance and lower adsorption energy with respect to the chemical adsorption. The adsorbed heavy metals have a significant effect on the electronic properties of the graphene flakes. The wide energy gap (similar to 3.7 eV) of the hexagonal flake decreases to 0.6 eV by surface-adsorption of hexahydrated Cd. This decrease occurs due to the inclusion of new energy states to the band structure which in turn leads to shifting of the highest occupied molecular orbital toward the Fermi level. The effect on the tiny band gap of the triangular flake is very small, the highest occupied and lowest unoccupied molecular orbitals are unaffected by the adsorption process. The calculated total charge on the hydrated metals implies that the charge transfer strongly depends on the interaction scheme. The lowest charge transfer from tetrahydrated Pb to the triangular flakes is observed in case of surface adsorption, Q = -0.05 (e), and the largest charge transfer, Q = 1.1 (e), is observed in the adsorption of tetrahydrated Cd on 2NH-functionalized triangular flake.